input. That’s the easy explanation!
The assembly should be fairly straightforward, as there
are not that many components. Take the usual precautions
to check component polarity and orientation. A protection
diode, D3 1N4004, was added at the last minute; it could be
bypassed/bridged, if you like. There are microphone mounting pads beside each input screw terminal pad, but you
might like to use fine wire to mount the microphone until
you work out the sensitivity you need. If you need maximum
sensitivity, mount the mic off the printed circuit board to
prevent the relay vibration from false triggering itself. If a
nine mm pot is unavailable, use a five mm trimmer pot.
Testing the Sound Activated
All testing should be done with C1 (1 µF) omitted;
this will make it easier to see what’s going on, as VR1 set
to about three-quarter-clockwise position should give
maximum sensitivity. Rotating it clockwise a little bit will
fully toggle on the LM358 output, while rotating counterclockwise will give a gradual gain reduction due to R8.
Without it you would get a drastic gain reduction that
would instantly snap OFF.
Once IC1 goes briefly high, C1 holds this voltage for
a second or so, preventing the relay from trying to switch
at audio frequencies. C1 also helps smooth out any relay
chatter that can occur when using an LDR at the transition point; an incandescent light source can cause this.
When setting up the microphone version particularly
near the very maximum sensitivity end of the scale, move
VR1 slowly and allow a second or so for the circuit to settle for best results. The sound activated relay version was
not designed to activate with constant low-level speech,
as this would make the circuit unusable in a practical
sense (it would be constantly going on or off). The circuit
needs a slight audio peak to trigger. The best response is
from percussion-type sounds mostly — clicks, pops,
thumps, etc. In some cases, the relay clicking on and off
may re-trigger itself. Vibration
travels fairly well through a circuit board; I have had the relay
coil singing to the microphone
in a feedback loop. The sensitivity of the circuit may be
increased slightly by mounting
the microphone off the circuit
If you need to have the sensor or microphone some distance
away from the circuit board, you
should use shielded wire to avoid
any noise induced into the cable.
The pad below R6 can be used for
the shield wire in this case.
NUTS & VOLTS
100Ω, see text
22 KΩ mic only or 100KΩ, see text
100 KΩ or link, see text
10 MΩ sound option only
1 MΩ or wire link for thermistor/LDR/voltage
100 KΩ 9 mm pot or 5 mm trimmer linear — RadioShack Cat 271-284 (5mm)
100 KΩ NTC or see text — RadioShack 10 KΩ NTC Cat 271-110
ORP12 or equiv. — RadioShack Cat 276-1657
1 µF electrolytic, see text
0.22 µF monolithic, mic input only — RadioShack 1.0 µF Cat 272-109
1N4004 — RadioShack Cat 276-1103
1N4148 — RadioShack Cat 276-1122
2N7000 N-channel MOSFET or equivalent
Color, your choice
Electret microphone, two-wire — RadioShack Cat 270-90
DPDT 12 V DC relay or 5 V, 6 V, 24 V DC if available
PCB mount screw terminals, 1 x 6 or 2 x 2 for professional looking job.
Clinch universal relay PCB
To fit the project in a small box, a low-profile relay is needed; they are available at slightly
higher cost. You may also need to do a small amount of filing near the PCB mounting holes to clear
the internal lid mounting pillars of the box. The VR1 pot needs a small hole drilled in the lid to protrude through or cut its shaft down slightly, but be careful. You may also want a hole for the LED.
If you’re using an IC socket, please use a machined pin type. Six- and 24-volt relays are also
available; with a few resistor changes, you could run the board at five, six, or 24 volts DC.
If you feel the need to correctly terminate the unused op-amp in the LM358, you could have
a look at this web page: www.maxim-ic.com/appnotes.cfm/appnote_number/1957
The components are commonly available at the usual electronic suppliers. I can supply a
quality PCB with top overlay and solder mask for the project.
Price: PCB $8.00 each, including GST, plus $4.00 P&H in Australia. Prices may come down;
check website for availability.
As mentioned earlier, try to
get a balanced resistor/sensor
resistance. This will give you a
better ± rotation (midway) on
VR1 for fine-tuning later. As a
starting point, you may like to
measure the resistance of the
LDR or thermistor at the temperature or light level you want
the relay to activate at, then
pick a resistor that is close to
If you happen to end up
with a very low resistance on